There exists a class of machinery which utilizes mechanical resonance as the means to obtain periodic motion of the machine elements. Reciprocating compressors of this class, often referred to as "resonant piston compressors," can be advantageously used in a variety of applications, such as for example, electrically-driven heat pump systems and the like.
In known free piston-type resonant reciprocating compressors the fluid compressing member, such as a piston, is driven by a suitable motor, such as a linear reciprocating electrodynamic motor. A compression piston is usually coupled to the motor armature and the armature held in a rest position by way of one or more main or resonance spring means. When the motor is energized, such as by an alternating current, a periodic magnetic force is generated to drive the piston. If the frequency of the magnetic force is sufficiently close to the mechanical resonance frequency of the compressor (as determined essentially by the mass of the reciprocating assembly and the combined stiffness of all mechanical and gas-spring components), the piston will oscillate back and forth to provide compression of the fluid.
U.S. Pat. Nos. 3,937,600 to White for a "Controlled Electrodynamic Linear Compressor" and 4,353,220 to Curwen for a "Resonant Piston Compressor Having Improved Stroke Control for Load-Following Electric Heat Pumps and the Like" relate to double-ended two-compressor-cylinder electrodynamic motor-driven resonant reciprocating compressors including gas springs. In such double-ended two-compressor-cylinder arrangements, identical parallel flow cylinders are involved. In principal, these two-compressor-cylinders would undergo the same compression cycle and would be subjected to the same pressure forces so that such design would (in theory) be intrinsically pressure balanced. In practicality, however, such designs are inherently unstable. As long as the two cylinders operate with the same value of mid-stroke volume (or equivalently, at the same clearance volume ratio) then the two cylinders will impose equal but oppositely-directed (cancelling) average pressure forces on the plunger-driven pistons. However, any slight offset bias of the plunger from the theoretical center position causes the average pressure forces on the two pistons to be unbalanced in such a way that it tends to push the plunger further off center, resulting in an axially unstable arrangement. To solve such a situation, these patents introduce ports on the gas springs. When the piston begins to go off center, an opposing average pressure force which is larger than the destabilizing force coming from the cylinder would be generated resulting in a stable operating center position.
Mechanical springs can be used for centering and axial stabilization but this increases weight, and unless the operating stroke is kept short can decrease the operating life due to spring fatigue.
In the ideal form, a free-piston-type resonant reciprocating machine has no mechanical connection between the reciprocating and stationary assemblies. Accordingly, prior to start-up the reciprocating assembly may be located anywhere between the mechanical overstroke limits, unless some special means is provided to lock the assembly at or close to its mid-stroke position. If no such means is provided, and if the axis of the reciprocating assembly is parallel to the earth's gravity axis, the reciprocating assembly will always be resting at the lower mechanical limit stop prior to start-up.
In general, a linear electric drive motor can often be employed which can produce a strong enough electromagnetic or electrodynamic centering force to bring the reciprocating assembly to the center position at start-up. This is often the most desirable solution to the start-up centering situation.
Use of mechanical centering springs can also be employed to solve the centering problem. Since the same mechanical spring means can also provide a means for achieving axial stability, such an approach is attractive. One major problem with mechanical springs, however, is that in order to obtain reasonable operating life the operating stroke must be kept small. That is, the problem of spring fatigue life always results in a restriction on the maximum operating stroke which can be used for the compressor and motor system. At short stroke operation there are severe combined penalties of motor cost, weight, size and efficiency.
While the arrangements of U.S. Pat. Nos. 3,973,600 and 4,353,220 have proved eminently satisfactory in the two-compressor-cylinder arrangement it is desirable to avoid the use of gas springs, since they introduce undesirable losses and contribute to increased manufacturing cost. It is desirable that further work be carried out to reduce the cost, weight, size, and to increase the efficiency of such machines.